1. Technical Field
The present invention relates to flat panel display screens, and more particularly to a flat panel display that utilizes a plurality of microgrooves having varying groove density.
2. Description of Related Art
Lighting systems which illuminate liquid crystal displays (LCDs) are well known. Typically, these lighting systems are used in portable electronics, such as lap-top computer displays, where bright illumination provided by low-power is preferred.
Conventional LCDs often employ planar light guides having edge-lit end surfaces and have an etched diffusion back surface to reflect light towards the front viewing surface. Some less conventional LCDs employ light guides having edge-lit end surfaces and prismatic microgrooves which are parallel to each other and the edge-lit surfaces. The microgroove-type LCDs use the combination of edge-lighting and microgrooves to reflect light, which would have been totally internally reflected, towards the front viewing surface. However the microgroove arrangement is far less common than the etched diffusion arrangement due to the ease of fabrication of the etched surfaces as compared to the microgrooved surfaces, and the lack of uniformity of illumination caused by the microgrooves.
The microgroove back lighting arrangements that exist typically employ a linear light source, such as a cold cathode fluorescent lamp (CCFI), at one or two edges of light transmissive material. The microgrooves are cut in the back surface to allow light entering the light transmissive material at angles greater than the critical angle necessary for total internal reflection to be reflected out the front surface toward the LCD. In this way, light which would normally be totally internally reflected by the otherwise nongrooved back surface is reflected toward the front surface providing a brighter display.
FIG. 1 shows a microgroove arrangement where light guide 11 is used for back lighting a liquid crystal display (LCD) 12. The light guide includes a front planar surface 13 and an opposite back light extracting and reflecting surface 14 created by facets 16 formed by V-shaped microgrooves (grooves) 17. Grooves 17 extend across surface 14, and facets 16 of grooves 17 make groove angles 19 which typically range between 45 and 90 degrees. Microgroove depths often range between 2.5-10 micrometers and the width of light guide 11 is approximately 1 millimeter. In conventional back lighting displays, groove step 18 remains constant along the length of surface 14.
A light input surface 20a is located adjacent a standard light source 21. Light source 21 includes a cylindrical envelope 22 which houses a coaxial filament 23. Filament 23 radiates light in all directions as indicated by arrows 24. A U-shaped reflector 26 encloses the lamp and reflects the energy into light guide 11 in a plurality of directions. Reflector 26 may comprise a thin sheet of reflective material. A reflector 28 has a reflecting surface 29 which is place adjacent the faceted surface 14, and reflects the light from surface 14 back into light guide 11 where it emerges to back light the LCD display 12.
A light input surface 20b is depicted as being adjacent to a reflector 38 which reflects any light traveling through the light guide 11 back into the light guide 11 to further increase the efficiency of conversion of light from incandescent source 21 for back lighting the LCD. Alternatively, reflector 38 can be replaced with a second light source (not shown).
U.S. Pat. No. 5,485,354 to Ciupke et al., U.S. Pat. No. 5,442,523 to Kasima et al., and U.S. Pat. No. 5,390,276 to Tai et al. each disclose flat panel displays that employ microgrooves and edge-lighting to provide a back lit display similar to that discussed above. The microgrooves have very broad angular ranges and use the range of angles to reflect light input from the edge towards the front surface and LCD. However, all of these conventional back lighting devices depict microgrooves having a fixed groove density and specify no relationship between the groove density and lighting uniformity viewed from the front surface. Furthermore, no relationship is specified between groove density and the distance from the edge of the light guide where light is introduced. There is also no relationship specified between groove angle and the critical angle of the light guide at which total internal reflection occurs.
The groove density and groove angle parameters have been found to be very important to the efficiency and the luminous uniformity of the back light design. Efficiency can be defined as the ratio between the luminous flux entering the light guide through the input edges and the luminous flux emanating from the flat panel display. Luminous uniformity measures intensity of light emitted from the front surface compared to other locations on the front surface. However, none of the existing technology provides high efficiency and uniformity by defining the relationship between groove density and distance from the edge of the light guide for groove angles related to the critical angle of the light guide.
FIG. 2 is a graphical representation depicting luminance distribution across the front surface of a light guide which can be used to evaluate performance of the light guide. FIG. 2 has an X-axis showing distance in millimeters from input surface (at 0 mm) to a second input surface (at 160 mm). A Y-axis shows luminance measured in candellas/meters.sup.2 times steradian (Cd/m.sup.2 sr) which is analogous to the conventional measure of luminance.
A plot 200 is generally shown by small squares and illustrates the illumination emanating from a conventional back lighting device such as that described above in conjunction with FIG. 1. It should be noted that a localized peak luminance 202 is shown for plot 200 at approximately ten millimeters from input surface (0 mm), and another localized peak 204 occurs at approximately 155 millimeters from input surface (0 mm). Localized peak 204 results from a second light source (not shown) being placed at the input surface located at 160 mm. Thus, peak 204 is approximately 5 mm from input surface at 160 mm.
Plot 200 illustrates that with two input light sources 21 in a conventional configuration, the luminance emitted from surface 13 (FIG. 1) falls into a luminance valley 206 at approximately midway along the 160 millimeter length. Thus, although those viewing a typical back lighting device are likely to view the center area near by valley 206, these viewers see the least amount of luminance as compared to areas closer to peaks 202 and 204 which are near light sources 21 than valley 206. This results largely from the midpoint (80 mm) of surface 13 being equally far from each light source 21. The occurrence of valley 206 defines limits to the size of flat panel display screens since luminance values falling below certain acceptable standards for illumination occur at points near the midpoint of the screen. Some conventional systems satisfy the standards by introducing more power which raises the luminance of valley 206 to acceptable levels. However, practical limits to the amount of power that portable devices could provide has limited the size of portable flat panel display screens.
Accordingly, a need exists to increase luminance around the center area of a flat panel display.
Another need exists for maximizing luminance at a midpoint between light sources thus providing better illumination than conventional systems for areas of significant viewing attention.
Another need exists for an arrangement that provides increased uniformity of illumination at each point along the viewing surface of the back light display.
Yet another need exists for providing a light guide which is suitable for back lighting large panel displays with diagonal measurements larger than 345 millimeters or 13.8 inches.
There is also a need for an arrangement which optimizes the groove angle and groove density of a back lighting display to increase luminance viewed from the front surface.